The present invention relates to an optical head for recording, reproducing or erasing information in an optical memory device by irradiating a laser beam to a storage medium. And the invention further relates to a magnetooptical head for recording information in a magnetooptical memory device for reproducing or erasing the recorded information by irradiating a laser beam to a storage medium which is composed of a magnetic thin film.
It has been observed recently that the use of an optical disk or magnetooptical disk as a large-capacity memory device is attracting much attention. The substrate of such optical or magnetooptical disk is generally composed of glass or plastic material. And it is customary that acrylic or polycarbonate plastic is employed in most cases due to remarkable merits thereof in both productivity and handling facility. However, such plastic substrate has some disadvantages in the points that, as compared with a glass substrate, double refraction is prone to occur (particularly in polycarbonate) and the optical characteristics are inferior. Such occurrence of double refraction is liable to be induced especially in a plastic substrate produced by injection molding.
For reproducing signals in any optical disk of reproduction-only type, DRAW type or erasable type based on phase transition, there is adopted a technique of eliminating return light to a laser diode (LD) through an optical isolater which is constituted by a combination of a polarized beam splitter (PBS) and a quarter wavelength plate. The light emitted from the laser diode is passed through the polarized beam splitter so that P-wave light alone is linearly polarized and, in a structure where the optical axis of the quarter wavelength plate is set to form an angle of 45 degrees with the oscillation surface, the linearly polarized light is converted into circularly polarized light before being incident upon the substrate. The reflected light is passed through the quarter wavelength plate again to become S-wave linearly polarized light, which is then introduced to a photo detector by the beam splitter without being returned to the laser diode.
However, if double refraction takes place in the substrate, the light reflected from the substrate is elliptically polarized and therefore fails to become linearly polarized light even after passage through the quarter wavelength plate, whereby return light to the laser diode is generated to consequently bring about a problem that the quantity of light introduced to the photo detector is reduced.
Meanwhile in a magnetooptical disk, signal reproduction is performed by the use of linearly polarized light. And in case there is existent such double refraction in the substrate of the disk, the linearly polarized light is harmfully affected by elliptical polarization to eventually cause considerable deterioration of the reproduced signal quality.